1. Field of the Invention
The present invention relates to an anomaly monitoring method of an injection molding machine that can monitor anomaly in closing a back-flow prevention valve after performing measurements with a screw having a back-flow prevention valve installed.
2. Description of the Relevant Art
In general, an inline screw injection molding machine prevents back flow of molten resin at the time of injection by installing a back-flow prevention valve (ring valve) at the tip of the screw. In addition, a conventional practice in this kind of injection molding machine is to forcibly close the back-flow prevention valve by reverse rotating the screw by a tiny number of rotations (angle of rotation) after performing measurements by rotating (forward rotating) the screw.
For example, disclosed in Japanese Unexamined Patent Application No. 11 (1999)-240052 is an operation method of an inline injection molding machine wherein the amount of measured molten mix-kneaded resin to be injected is prevented from varying the amount at the time of suck back or injection, reducing the scatter in the amount of filling. In this operation method, after finishing the measurement of molten mix-kneaded resin stored in the tip of a heating cylinder, the screw is rotated in reverse to move a check ring until it makes contact with a check sheet, closing a resin path formed between the check ring and the check sheet, and afterwards a suck-back process is performed.
On the other hand, in normal injection molding, because measured resin includes a cushion amount, the amount of filling can be supplemented by adding a dwell pressure even if the amount of resin or density may somewhat change, providing a good product in the case of a general-use product or a molded product for which precision is not strictly demanded. However, in recent years, a high level of molding precision has come to be demanded due to miniaturization and higher precision of molded products. For example, a narrow pitch connector of several millimeters in size needs to be formed only by a filling process with no dwell pressure added in order to prevent deformation of an inserted connector pin, and in a thin optical disk formed by injection compressed molding, because the amount of filled resin directly causes unevenness of the thickness of the disk, a constant amount of resin always needs to be supplied to a cavity. Therefore, in forming such a precision molded product, securing a precise amount of filling (amount of resin) is always required, important in securing a precise amount of filling whether a back-flow prevention valve is completely closed or not. If closure is incomplete, back flow of resin occurs, and even if such back flow is a tiny amount, it will immediately cause scatter in the amount of filling.
In this manner, in forming a precision molded product, it is important to precisely monitor the closed state of a back-flow prevention valve. An injection molding machine which can check such closed state of a back-flow prevention valve is disclosed known in Japanese Unexamined Patent Application No. 63 (1988)-227316, which comprises an injection molding machine equipped with a back-flow prevention valve at the screw tip inside a heating cylinder, constituted by providing a communication means inside the screw which communicates a position change accompanying the opening/closing of the back-flow prevention valve to the outside, and a position detection means in rear of the screw which detects the position change communicated by the communication means.
However, the conventional injection molding machines which monitor (checks) the closed state of the back-flow prevention valve had the following problems.
First, because its construction comprises a communication means which communicates the position change of the back-flow prevention valve to the outside and a position detection means which detects the position change communicated by the communication means, there are many mechanical communication mechanisms, which limit precision in detecting a tiny displacement of the back-flow prevention valve, making if impossible to perform highly reliable or stable monitoring of whether the back-flow prevention valve is completely closed or not.
Second, because it requires the communication means inside the screw and the position detection means in rear of the screw, its structure becomes intricate and complex, with significant cost increase and scale increase accompanying the parts cost and manufacturing cost.